pith -
the choicest or most essential or most vital part of some idea or experience; "the gist of the prosecutor's argument"; "the heart and soul of the Republican Party"; "the nub of the story"
Pith is a substance that is found in vascular plants. It consists of soft, spongy parenchyma cells, and is located in the center of the stem in eudicots (both herbaceous and woody) and in the center of the roots in monocots. It is encircled by a ring of xylem, and outside that, a ring of phloem.